This invention relates to ion mobility detectors and more particularly the ion accelerating structure of such detectors.
Ion mobility detectors are used to detect the presence of unknown materials in an environment, for example contaminants in atmospheric air. A library of known possible contaminants is built up and the measurements known for these are then compared with the results with an unknown species to decide whether a sample contains a contaminant and if so whether it has already been identified. Measurement of concentration or an indication of concentration can be given as well as qualitative identification of the species.
Typical prior art ion mobility detectors have an ionization source, an ion reactant region, an ion drift region, an ion injection shutter or grid interposed between the ion reactant region and the ion drift region, and an ion detector. The systems operate at atmospheric pressure where the mean free path of the contained gas in the drift region is a small fraction of the dimensions of the container. A carrier gas, normally purified atmospheric air (particularly purified to remove water vapour which can interfere with certain types of charged species) is introduced into the ion mobility detector with a gaseous sample of the material whose identity is to be determined by characterization of its ion mobility properties. The carrier gas containing the sample is introduced through an inlet so as to be exposed to the ionization source. This causes portions of both the carrier gas and the sample to be directly ionized at the ionization source. In general, the molecules of the carrier gas are more easily ionized by the ionization source than other molecules of the sample. The gaseous mixture is located within the reactant region at this stage and since the mean free path is many times smaller than the dimensions of the reactor region, multiple collisions between the molecules of the carrier and the sample gas(es) occur, the result of which is that the ion charge tends to be transferred by these collisions from the carrier molecules to the sample molecules thus resulting in a secondary ionization process which ionizes an increased number of the molecules of the sample gas. The reactant region is normally arranged to be under the influence of a potential gradient which moves the charged mixture towards the ion injection grid which is electrically charged to prevent transfer of ions from the reactant region to the drift region but which can be de-energized so as to let a pulse of ions pass through into the drift region. Accordingly, periodically the grid is deionized for a short time and a number of ions are introduced into the drift region. The drift region is arranged to be under the influence of an electrostatic drift field or potential gradient which acts to move ions in the drift region down the tube away from the ion injection grid towards a collector grid which detects the charged ions and is located at the end of the drift region. The time of arrival of each ion at the detector grid relative to the time that the ion injection grid was opened is determined by the mobility of the ion in the nonionized gas occupying the drift region. Heavier ions move more slowly through the drift region and take longer to arrive at the detector than lighter ions. Ions with similar mobilities tend to bunch together and arrive in groups or bunches at the detector region producing a distribution curve and this can be used to characterize the ions, the peak of the curve being the average for that group or bunch of ions thus enabling one to determine the time taken between the opening of the grid and arrival of the group at the detector.
As mentioned above the present invention is concerned particularly with the structure of the drift region.
Commercially available ion mobility detectors are known which have a drift region of tubular configuration constructed of alternating rings of ceramic material and metal, these electrically conductive metal rings being called guard rings. The stack of rings is clamped together and sealed so as to make a gas tight tube. Since it is difficult to ensure a seal between the rings, conventional devices of this type often enclose the stack of rings within an outer sealed envelope. The electrostatic drift field potential gradient is established by connecting adjacent guard rings to each other via a resistor and connecting the end guard rings to the terminals of a voltage source. The conductive rings afford a series of ascending voltage levels and the longitudinal axis of the tube coincides with the longitudinal axis of the electrostatic field which is thus established.
U.S. Pat. No. 4390784 proposes a drift tube consisting of a tube of ceramic or glass or other suitable nonconductive material, coated continuously along its internal surface with a thick film resistor composition. The drift tube is 3.25" long with an internal diameter of 1" and a potential difference of 1500 volts is impressed across the tube. The patentees in U.S. Pat. No. 4390784 anticipated that the inner glass face of the tube afforded by the glass-like surface of the thick film resistor would "cause ions impinging on the overcoat to stick in the ionized state to the overcoat and neutralize and destroy the fields within the drift region". They found that this was not the case, but did not know why and suggested that either the glass-like surface of the thick film resistor was sufficiently conductive the allow the ions impinging on the interior of the tubes to be deionized and thus become ineffective in neutralizing the field or else the ions did stick to the surface but repelled like ions approaching the surface, the net change in the field thus being negligable. They thought it likely that their arrangement was operative because it included elements of both the above possible effects.
U.S. Pat. No. 3522425 discloses a "conventional" voltage divider 50 comprising a plurality of spaced circumferential conductive plates interconnected by resistors 52 and terminally connected to a lead 26 connectable to a battery source and to a grounded electrode 12 so as to provide a relatively uniform field gradient. The specific potential difference utilized in U.S. Pat. No. 3522425 was 625 volts.
GB 2033145A discloses a drift tube in which conductive rings are disposed on the inner surface of a non-conductive housing to establish a linear electric field along the tube. The rings are thicker than they are wide and are spaced apart by distances greater than their width. They are shown as connected by wires passing out through the housing to an external voltage divider network.